Variable gain low noise amplifier

ABSTRACT

This present invention is related to a variable gain low noise amplifier which is operated to the best operation of input matching, gain and noise characteristics, linearity. The variable gain low noise amplifier according to an embodiment of the present invention includes a first amplifying cell operated in high gain mode, second amplifying cell operated in low gain mode, selectively matching circuit, and fist short-circuit means. It is provided to the variable gain low noise amplifier according to the present invention for operating the best operation in each gain mode so that the circuit operated in high and low gain modes is not affect to a load with each other.

TECHNICAL FIELD

[0001] The present invention relates to Low Noise Amplifier (hereinafter called “LNA”), more specifically, it is related to a variable gain LNA that is operated most suitably in input matching, gain and noise characteristics, and linearity, etc.

BACKGROUND OF THE INVENTION

[0002] A first terminal is comprised to an amplifier that generally amplifies small signal to large signal in wireless equipments, for example a portable phone, TV etc. This amplifier is demanded to have an amplifying operation having characteristics of low noise and high gain when the signal is very small. But linearity is demanded rather than the amplifying operation when the signal is relatively large. Therefore, the amplifier satisfies amplifying mode of more than two kinds according to input signal level, and it is necessary that the amplifier should be selected one of them, in the wireless frequency receiving equipments.

[0003] As a low noise amplifier in the prior arts, it is disclosed in U.S. Pat. No 6,144,254 that it is possible to switching both a low gain and high gain state.

[0004]FIG. 1 shows a circuit diagram of a low noise amplifier disclosed in U.S. Pat. No. 6,144,254.

[0005] As shown in FIG. 1, the low noise amplifier comprises a common-emitter BN1 (the first NPN transistor to operate in a high gain state), common-base BN2 (the second NPN transistor to operate in a low gain state), third NPN transistor BN3 (the third NPN transistor for provide bias current in BN2) and resistor R1.

[0006] That is, a collector of the first NPN transistor BN1 is connected to an output terminal Pout of a LNA, a base is connected to an input terminal Pin of a LNA and the first bias input terminal Bias1, and an emitter is grounded. The resistor R1 is connected between the first bias input terminal Bias1 and first NPN transistor BN1.

[0007] A collector of the second NPN transistor BN2 is connected to the output terminal Pout of LNA, a base is connected to the second bias input terminal Bias2, and an emitter is connected to the input terminal Pin of LNA and a collector of the third NPN transistor BN3.

[0008] A base of BN3 is connected to Bias 3 (the third bias input terminal), and an emitter is grounded.

[0009] Hereinafter describes an operation of a LNA in the prior arts, referring to FIG. 1.

[0010] In the high gain state, Bias 1 is high, and Bias 2 and Bias 3 are low. Therefore, BN1 is activated and performs the amplifying operation of a high gain, in the high state. Here, BN 2 and BN 3 are turned off.

[0011] In the low gain state, Bias 2 and Bias 3 are high, and Bias 1 is low. Therefore, BN2 and BN3 are activated and perform the amplifying operation of low gain, in the low gain state. Here, BN 1 is turned off.

[0012] The low noise amplifier shown in FIG. 1 selects one of states between high gain and low gain, and then operates the high-gain or low-gain amplifying operation in accordance with a size of the received signal. But, the circuits operated to each gain state is affect to a load with each other, because input terminals of the emitter-common first NPN transistor BN1 and base-common second NPN transistor BN2 are directly connected with each other, that is the base of BN1 is directly connected with a emitter of BN2, in the low noise amplifier shown in FIG. 1. Namely, when the low noise amplifier operates in high gain state, the capacitance of emitter terminal of BN 2 acts as a load of high gain circuits. As a result of that, the gain, matching and the noise characteristic of high gain state are not good, and the capability of the low noise amplifier is reduced. In addition, when low noise amplifier operates in low gain state, the capability of low gain state is reduced by the capacitance of the base terminal of BN1, in the same with high gain state. Because two mode impedance level of input terminal are substantial same level, and the impedances act as a load with each other.

SUMMARY OF THE INVENTION

[0013] The object of the present invention is to provide a variable gain low noise amplifier that the circuits designed to operate in the best suited in each gain mode is not affected the capability of the best suited to operate in the other gain mode.

[0014] Another object of the present invention is to provide variable gain low noise amplifier that input matching, gain, noise characteristics, linearity, etc can be displayed the capability of the best suited in each other different mode.

[0015] The other object of the present invention is to provide variable gain low noise amplifier which is operated according to the size of receiving signal in more than two amplifying mode, and can be varied the gain in low gain mode.

[0016] The other object of the present invention is to provide variable gain low noise amplifier that power consumption is low.

[0017] According to achieve above objects, a variable gain low noise amplifier, which amplifies the signal applied in an input terminal and outputs to an output terminal, comprises a first amplifying cell, which comprises a first terminal and second terminal connected to the output terminal, amplifies the signal applied to the first terminal to high gain, and outputs to the second terminal in high gain mode; a second amplifying cell, comprises a first terminal and second terminal connected to the output terminal, amplifies the signal applied to the first terminal to low gain, and outputs to the second terminal in low gain mode; a selectively matching circuit, comprises a first terminal connected to the input terminal and second terminal connected to the first terminal of the first amplifying cell, and selectively changes an input impedance of the first amplifying cell; a first short-circuit means connected between the input terminal and the first terminal of the amplifying cell, and transmits the signal applied to the input terminal to the first terminal of the second amplifying cell in the operation of low gain mode; and wherein the selectively matching circuit changes the input impedance that the power transmitted to the first amplifying cell of the signal applied to the input terminal is to be maximum in the operation of high gain mode, and to be minimum of essentially zero in the operation of low gain mode.

[0018] The variable gain low noise amplifier of the present invention, further comprises a short-circuit means connected between the second terminal of the second amplifying cell and the output terminal.

[0019] The variable gain low noise amplifier of the present invention, further comprises a short-circuit means connected between the input terminal and the output terminal.

[0020] The variable gain low noise amplifier of the present invention, wherein the first amplifying cell comprises first, second, third terminals, amplifying element, resistor and degradation impedance, and wherein the amplifying element, resistor and degradation impedance are controlled the amounts of current flowed from the first terminal to the second terminal in proportion to the voltage applied to the third terminal; and a first terminal of the amplifying element is formed to the second terminal of the first amplifying cell, the second terminal is connected with one of terminals of the degradation impedance, the third terminal is connected with one of terminals of the resistor and then formed to the first terminal of the first amplifying cell, the other terminal of the resistor is applied to the HG-bias voltage of activating the first amplifying cell in an operation of high gain mode, the other terminal of the degradation impedance is grounded, and the amplifying element is connected to common mode of the second terminal.

[0021] The variable gain low noise amplifier of the present invention, wherein the second amplifying cell comprises a first, second, and third terminals; a first amplifying element controlled the amounts of current flowed from the first terminal to the second terminal in proportion to the voltage applied to the third terminal; wherein the second terminal of the first amplifying element is formed to the first terminal of the second amplifying cell, and the third terminal is applied to the LG-bias voltage of activating the second amplifying cell in the operation of low gain mode, and the first amplifying element comprises an amplifying unit connected to common mode of the third terminal; and second and third amplifying element, voltage source, and variable voltage source, which are controlled the amounts of current flowed from the first terminal to the second terminal in proportion to the voltage applied to the third terminal; wherein the first terminal of the second amplifying element is formed to the second terminal of the second amplifying cell, the second terminal is connected to the first terminal of the first amplifying element of the amplifying unit by connecting with the second terminal of the third amplifying element, the third terminal is connected with one of the terminals of the voltage source, the first terminal of the third amplifying element is connected to the power source, the third terminal is connected to the variable voltage source, and the other terminals of the voltage source and variable voltage source are grounded.

[0022] A variable gain low noise amplifier of the present invention, wherein the matching circuit comprises a first and second inductor, capacitor, and short-circuit means; and one of the terminals of the first inductor is connected with the second inductor and the capacitor, the other terminal is connected to the short-circuit means, the other terminal of the second inductor is formed first terminal of the matching circuit, the other terminal of the capacitor is formed to second terminal of the matching circuit, and the other terminal of the short-circuit means is grounded.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 shows a circuit diagram of a low noise amplifier in the prior arts.

[0024]FIG. 2a shows a circuit diagram of a source-common amplifier in the prior arts.

[0025]FIG. 2b shows a circuit diagram of a gate-common amplifier in the prior arts.

[0026]FIG. 3 shows a circuit diagram of a variable gain LNA according to an embodiment of the present invention.

[0027]FIG. 4 shows a circuit diagram of a variable gain LNA according to another embodiment of the present invention.

[0028]FIG. 5a shows a circuit diagram of a first amplifying cell according to an embodiment of the present invention as the variable gain LNA showed in FIG. 3 and FIG. 4.

[0029]FIG. 5b shows a circuit diagram of a second amplifying cell according to an embodiment of the present invention as the variable gain LNA showed in FIG. 3 and FIG. 4.

[0030]FIG. 5c shows a circuit diagram of a selectively matching circuit according to an embodiment of the present invention as the variable gain LNA showed in FIG. 3 and FIG. 4.

[0031]FIG. 6a shows a circuit diagram of the variable gain low noise amplifier shown in FIG. 3 using circuits shown in FIG. 5a, 5 b and 5 c.

[0032]FIG. 6b shows an equivalent circuit diagram of the input part of the first amplifying cell in order to describe the operation of selective matching circuit in accordance with an embodiment of the present invention, when the variable gain low noise amplifier is operated in high gain mode.

[0033]FIG. 6c shows an equivalent circuit diagram of the input part of the first amplifying cell in order to describe the operation of selective matching circuit in accordance with an embodiment of the present invention, when the variable gain low noise amplifier is operated in low gain mode.

DETAILED DESCRIPTION

[0034] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings.

[0035] Here, a common source and gate low noise amplifier of prior arts will be described, and then proper embodiments of a variable gain low noise amplifier according to present invention will be described in detail with reference to the attached drawings.

[0036]FIG. 2a shows a common source amplifier in prior arts.

[0037] Showing in FIG. 2, a common source amplifier comprises NMOS transistor MS21, first inductor L21, second inductor L22 and third inductor L23, resistor R21 and voltage source V21. The drain of NMOS transistor MS21 is formed to an output terminal Pout connected with one of terminals of first inductor L21, the gate is connected with resistor R21 and third inductor L23, and the source is connected with one of terminals of second inductor L22. The other terminal of first inductor L21 is connected to a power source VDD, and the other terminal of second inductor L22 is grounded, and the other terminal of third inductor L23 is formed to an input terminal Pin of the amplifier. The voltage source V21 is connected between the other terminal of the resistor R21 and ground.

[0038] The common source amplifier showed FIG. 2a amplifies the signal applied through the input terminal Pin to high gain, and can be matched the input power and noise through a degradation of source. Accordingly, the source common amplifier is proper to get a maximum noise characteristic and gain. But, it is a weak point that linearity is bad due to a voltage amplification effect of caused by capacitance of third inductor L23 and NMOS transistor MS21.

[0039]FIG. 2b shows circuit diagram of gate common amplifier in prior arts.

[0040] Showed in FIG. 2b, the gate common amplifier comprises a NMOS transistor MG21, an inductor L24, a capacitor C21 and a current source 121. The drain of the NMOS transistor MG21 is formed to an output terminal Pout of the amplifier connected with one of the terminals of the inductor L24, the gate is connected to one of the terminals of the voltage source V22, and the source is connected with one of the terminals of the current source 121 and capacitor C21. The other terminal of the voltage source V22 is grounded, the other terminal of the inductor L24 is connected to the power VDD and the other terminal of the current source 121 is grounded. The other terminal of the capacitor C21 is formed to an input terminal Pin of the amplifier.

[0041] The gate common amplifier showed in FIG. 2b amplifies the signal applied through the input terminal Pin to low gain. Because it cannot be get an effect of voltage amplification to use an input matching circuit in the gate common amplifier, the gain and noise characteristic is bad to compare with the source common amplifier showed in FIG. 2a. But, it is easy to match an input resistance through the gm value by controlling of a current flowed in the transistor NMOS, because the input resistance is 1/gm. The value of the input resistance is much smaller than that of the source common amplifier showed in FIG. 2a. It is a strong point that the gate common amplifier can get a very high linearity more than the source common amplifier, because the linearity of gm is superior in case of a recently developed transistor having small channel length. And it is easy for the gate common amplifier to operate a variable gain function by means of addition of the circuit which is to vary the output current in the output terminal of the amplifier.

[0042] Hereinafter describes an embodiment of a variable gain low noise amplifier (LNA).

[0043] The variable gain LNA according to the present invention makes the use of an amplification element that is a MOSFET transistor. The amplification element comprises a gate, source and drain. The MOSFET transistor has the characteristics that amounts and the direction of a current flowed from the source to the drain or the other way is decided according to the value and polarity of the voltage applied to the gate. Other amplification elements like the MOSFET are a bipolar junction transistor (BJT), junction field effect transistor (JFET), metal oxide semiconductor field effect transistor (MOSFET), metal semiconductor field effect transistor (MESFET), etc.

[0044] Hereinafter describes mostly the MOSFET among the above amplification elements. But the sprit and scope of the present invention is not limited to the MOSFET element and may be applied all the other equivalent elements. And hereinafter describes mostly an N type MOSFET, but it is obvious to those skilled in the arts that the spirit and scope of the present invention may be applied to a P type MOSFET not limited to the N type MOSFET.

[0045]FIG. 3 shows a circuit diagram of a variable gain LNA according to an embodiment of the present invention.

[0046] Showed in FIG. 3, the variable gain LNA according to an embodiment of the present invention comprises a first amplifying cell 3100, second amplifying cell 3300, selectively matching circuit 3500 and first means of short circuit SW1. The first amplifying cell 3100 comprises a first terminal 301 and second terminal 303, and amplifies a signal applied to the first terminal 301 to high gain as minimize an additional noise, in high gain mode. The second amplifying cell 3300 comprises a first terminal 305 and second terminal 307, and amplifies a signal applied to the first terminal 305 as control the gain, in low gain mode. The selectively matching circuit 3500 comprises a first and second terminal 309, 311 and selectively changes an input impedance of the first amplifying cell 3100 for that the circuit operated in each gain mode does not act on a load respectively.

[0047] Hereinafter describes the connection relation of those, referring to FIG. 3.

[0048] The first terminal 301 of the first amplifying cell 3100 is connected with the second terminal 311 of the selectively matching circuit 3500, and the second terminal 303 is connected with the second terminal 307 of the second amplifying cell 3300 and formed to an output terminal Pout of LNA. The first terminal 305 of the second amplifying cell 3300 is connected with the first short-circuit means SW1. The first terminal 309 of the selectively matching circuit 3500 is connected with the other terminal of the first short-circuit means SW1 and formed to an input terminal Pin of LNA.

[0049] Showed in FIG. 3, the third short-circuit terminal may comprised between the second terminal 307 of the second amplifying cell 3300 and the output terminal Pout in the variable gain LNA according to an embodiment of the present invention. And that, the output signal of the second amplifying cell 3300 outputs to the output terminal of the LNA due to the third short-circuit means SW3 in the operation of the low gain mode.

[0050] Hereinafter describes an operation of the variable gain LNA according to an embodiment of the present invention.

[0051] The variable gain LNA is operated in two mode, that is high gain and low gain mode, according to a power level of a received signal. That is, it is operated in high gain mode when the power level of the received signal is under the threshold power decided beforehand, and operated in low gain mode when the power level of the received signal is over the threshold power.

[0052] The short-circuit means may reduce the signal due to having a disregardless resistance value in the state of short-circuit, and may be operated to a load due to having a finiteness reactance value in the state of open-circuit, when it is operated in high gain mode. Accordingly, it must restrain to use the short-circuit means in the circuit operated in high gain mode, and the above load characteristics of the short-circuit means must be carefully considered in case of each amplification circuit operation respectively.

[0053] When it is high gain mode, the first amplification cell 3100 is activated by that the first short-circuit means SW1 is opened and HG-bias is applied to the first amplifying cell 3100. And the second amplifying cell 3300 is inert by non-applying LG-bias.

[0054] Accordingly, when it is operated in high gain mode, the impedance of the second amplifying cell 3500 operated in low gain mode is not effect to the first amplifying cell 3100 operated in high gain mode by opening of the first short-circuit means SW1, and only the open impedance of the first short-circuit means is effect to the high gain mode circuit. But, the input of the first amplifying cell 3100 operated in high gain mode is generally matched to 50˜70 ohm, that is standard resistance, due to the selectively matching circuit 3500, the open impedance of the first short-circuit means SW1 is so high value more than 50˜70 ohm. So, the effect that the open impedance is to a load for the first amplifying cell 3100 is ignored. Accordingly, the variable gain LNA according to an embodiment of the present invention may be the most suitable operation in high gain mode, and amplifies the input signal to high gain.

[0055] When it is low gain mode, the second amplifying cell 3300 is activated by that the first short-circuit means is shorted and LG-bias is applied. And, the first amplifying cell 3100 is inert by non-applying HG-bias. The selectively matching circuit 3500 changes the input impedance of the first amplifying cell 3100 in low gain mode to high impedance more than the beforehand decided value (generally standard resistance value: 50˜75 ohm). Accordingly, in low gain mode the variable gain LNA may be the most suitable operation because the first amplifying cell 3100 is not operated to a load of the second amplifying cell 3300 in low gain mode.

[0056]FIG. 4 shows a circuit diagram of a variable gain LNA according to another embodiment of the present invention.

[0057] The embodiment showed in FIG. 4 is different to the variable gain LNA according to the embodiment showed in FIG. 3 in the point that fourth short-circuit means SW4 is comprised between the input terminal Pin and output terminal Pout. The variable gain LNA according to another embodiment of the present invention directly transmits the received signal to the output terminal Pout through the fourth short-circuit means. So, power consumption may be reduced according to this embodiment. Moreover high linearity is provided and signal distortion is reduced by eliminating an input signal level of post part (normally mixer) of the variable gain LNA.

[0058]FIG. 5a shows a circuit diagram of a first amplifying cell according to an embodiment of the present invention in the variable gain LNA showed in FIG. 3 and FIG. 4.

[0059] As shown in FIG. 5a, the first amplifying cell 3100 is embodied in common-source, and comprises an amplifying element MS51, degradation impedance DI51, and resistor R51. The drain of the amplifying element MS51 is formed to the second terminal 303 of the first amplifying cell 3100, the gate is connected to the terminal of the resistor R51 and is formed to the first terminal 301 of the first amplifying cell 3100, and the source is connected to a terminal of degradation impedance DI51. In the other terminal of resistance R51, when high gain mode operates, the first amplifying cell 3100 is activated by HG-biasing voltage, the other terminal of degradation impedance DI51 is grounded. Degradation impedance DI51 can be embodied to use the passive or the active elements of resistor and inductor, etc.

[0060] As mentioned above, the common-source amplifier has excellent noise and gain characteristics, and can get satisfying input power and noise matching at the same time, through the degradation impedance DI51 connected to the source of the amplifying element MS51. Also, as shown in FIG. 3, the common-source amplifier can be displayed the capability of the best suited in noise and gain side, because matching circuit 3500 is connected to the first terminal of the first amplifying cell 3100. Therefore, when the low noise amplifier is used to the above common-source amplifier in the high gain mode which need high gain amplifying operation, it can be displayed the most capability.

[0061]FIG. 5b is a circuit diagram for showing inside structure of the second amplifying cell 3300 in accordance with an embodiment of the present invention, in variable gain low noise amplifier shown in FIG. 3 and FIG. 4.

[0062] As shown in FIG. 5b, the second amplifying cell 3300 is embodied in common-gate, and comprises an amplifying part 510 and gain part 530.

[0063] The amplifying part 510 of the second amplifying cell 3300 comprises the first amplifying element MG51. The drain of the first amplifying element MG51 is connected to the connecting point of the source of the second and third amplifying element MG52, MG53 of variable gain part, and when operating low gain mode, LG-biasing voltage to be activate the second amplifying cell 3300 is driven, and source is formed the first terminal 305 of the second amplifying cell 3300.

[0064] In variable gain low noise amplifier in accordance with an embodiment of the present invention, preferably, current source (be not shown, refer to FIG. 2) is provided between the source of the first amplifying element MG51 and ground. In this case, it can be changed trans-conductance (gm) data of the first amplifying element MG51, and it can be controlled input impedance data of amplifying part 510, by control of current source data.

[0065] A variable gain part 530 comprises the second and the third amplifying elements MG52, MG53 and voltage source V51 and variable voltage source V52. The drain of the second amplifying element MG52 is formed the second terminal 307 of the second amplifying cell 3300, and gate is connected to one of terminal of voltage source V51, source is connected to the third amplifying element MG53. The drain of the third amplifying element MG53 is connected to power source VDD, gate is connected to variable voltage source V52. The drain of the third amplifying element MG53 is connected to power source VDD, gate is connected to variable voltage source V52.

[0066] An amplifying part 510 comprises the first amplifying element MG51 connected to common-gate, and amplifies the signal driven in the first terminal 305 of the second amplifying cell 3300. As mentioned above, common-gate amplifier can easily controlled input matching, and it has an excellent linearity.

[0067] As the variable gain part 530 controls the current quantities which divided the current came from amplifying part into the second and the third amplifying elements MG52, MG53, by the control of variable voltage source V52, the variable gain part 530 can vary the output came from the second terminal 307 of the second amplifying cell 3300, and it can control in succession the gain data of the second amplifying cell 3300. In addition, because the current of the first amplifying element MG51 is not changed by variable gain part 530, the trans-conductance data of the first amplifying element MG51 is regular, the input matching of the second amplifying cell 3300 is not changed.

[0068] Therefore, if variable gain part is used the second amplifying cell 3300 which has common-gate construction shown in FIG. 5b in low gain mode, it can perform successive variable gain function without changing the characteristic of input matching, and a low noise amplifier can be gotten a excellent linearity.

[0069]FIG. 5c is a circuit diagram for showing selective matching circuit 3500 in accordance with an embodiment of the present invention, in variable gain low noise amplifier shown in FIG. 3 and FIG. 4.

[0070] As shown in FIG. 5c, the selective matching circuit 3500 in accordance with an embodiment of the present invention comprises the first and the second inductor L51, L52 and capacitor C51 and the second short circuit-means SW2.

[0071] One of the terminals of the first inductor L51 is connected to the second inductor L52 and capacitor C51, the other terminal is connected to one of the terminals of the second short circuit-means SW2. The other terminal of the second inductor L52 is formed the first terminal 309 of selective matching circuit 3500, the other terminal of capacitor C51 is formed the second terminal 311 of selective matching circuit 3500, the other terminal of the second short circuit-means SW2 is grounded.

[0072] In the selective matching circuit 3500 in accordance with an embodiment of the present invention, the second short circuit-means SW2 can be displayed the capability of the best suited in the state of the first amplifying cell 3300 is activated, by high gain mode driven HG-biasing; in case of low gain mode, i.e., as HG-biasing is not driven, the selective matching circuit 3500 is selected the input impedance of the high gain mode circuit block which is made selective matching circuit 3500 and the first amplifying cell 3100, in case the first amplifying cell 3100 is not activated.

[0073]FIG. 6a shows a circuit diagram of the variable gain low noise amplifier shown in FIG. 3 using circuits shown in FIG. 5a, 5 b and 5 c.

[0074] As shown in FIG. 6a, variable gain low noise amplifier in accordance with an embodiment of the present invention comprises the first and second amplifying cell 3100, 3300 and selective matching circuit 3500 and the first short circuit-means SW1.

[0075] The first amplifying cell 3100 is embodied in the form of common-source and is operated in high gain mode, and the second amplifying cell 3300 is embodied in the form of common-gate and is operated in low gain mode. Moreover, selective matching circuit 3500 comprises the second short circuit-means, and in high gain mode, the input of the first amplifying cell 3100 is matched by selective matching circuit 3500 in order that the first amplifying cell 3100 can be displayed the capability of the best suited; and in low gain mode, the input impedance of the first amplifying cell 3100 is changed in a high data by selective matching circuit 3500, and the first amplifying cell 3100 is not operated as the load of the second amplifying cell 3300. Therefore, variable gain low noise amplifier in accordance with an embodiment of the present invention can be displayed the capability of the best suited in each gain mode.

[0076] Also, variable gain low noise amplifier in accordance with an embodiment of the present invention is used common-source the first amplifying cell 3100 in high gain mode demanded high noise and gain characteristic, and is used common-gate the second amplifying cell 3100 in low gain mode demanded high linearity and successive variable gain, as a consequence of that, a linearity is good and input matching is easy and amplifier can be embodied variable gain low noise amplifier which is possible successive variable gain function.

[0077] Furthermore, in case the power level of receiving signal is enough large and amplifying is not need, as receiving signal is directly passed in output terminal by the fourth short circuit-means SW4 shown in Fig, power consumption demanded for amplifying operation is not need.

[0078]FIG. 6b and FIG. 6c are circuit diagrams shown in equivalent the input part of the first amplifying cell 3100 in order to described more specifically the operation of selective matching circuit 3500 in accordance with an embodiment of the present invention, in case variable gain low noise amplifier is operated in each high gain mode and low gain mode.

[0079] In case of high gain mode, the second short circuit-means is open, and the input of activated the first amplifying cell 3100 can be shown in a equivalent ZHG,on. At this time, variable gain low noise amplifier can be gotten the capability of the best suited by matching the input impedance of the first amplifying cell 3100 by means of using the second inductor L52 and capacitor C51. That is, because the electric powers of variable gain low noise amplifier and the first amplifying cell 3100, the high gain mode characteristic of the best suited can be gotten.

[0080] In case of low gain mode, the second short circuit-means is short, and inactivated the input of the first amplifying cell 3100 can be shown in a equivalent ZLG,off. Here, ZLG,off is had views on to be very different ZHG,on′data, this time, matching circuit 3500 comprises the first and the second inductor L51, L52 and capacitor C51. The first inductor L51 has the inductance data that the input part of the first amplifying seems the substantial infinite input impedance in node 309. As it does this way, in case of low gain mode, the input part of the first amplifying cell 3100 is not affected in the second amplifying cell 3300. Therefore, the input of variable gain low noise amplifier is matched in the most fitted the input of the second amplifying cell 3300, because the maximum power Pin is transferred, and at the same time, the power of the first amplifying cell 3100 is substantial zero, variable gain low noise amplifier is gotten the low gain mode characteristic of the best suited.

INDUSTRIAL APPLICABILITY

[0081] In a low noise amplifier in accordance with the present invention, because the operating circuits in each gain mode is not affected in the performance of the operating circuit of the best suited in the other gain mode, the each circuits can be displayed the capability of the best suited in the each mode.

[0082] Also, input matching, gain, noise characteristic and linearity etc, can be displayed the capability of the best suited in the each other different gain mode.

[0083] Further, the each circuits is operated in gain mode more than two according to the size of receiving signals, and can be varied gain in low gain mode.

[0084] Furthermore, because the output of each circuits get to be equal to the receiving signals by the short circuit-means, in case amplifying operation is not need, the power consumption is reduced. 

In the claims:
 1. A variable gain low noise amplifier, amplifies the signal applied in an input terminal and outputs to an output terminal, comprising: a first amplifying cell, comprises a first terminal and second terminal connected to said output terminal, amplifies said signal applied to said first terminal to high gain, and outputs to said second terminal in high gain mode; a second amplifying cell, comprises a first terminal and second terminal connected to said output terminal, amplifies said signal applied to said first terminal to low gain, and outputs to said second terminal in low gain mode; a selectively matching circuit, comprises a first terminal connected to said input terminal and second terminal connected to said first terminal of said first amplifying cell, and selectively changes an input impedance of said first amplifying cell; a first short-circuit means connected between said input terminal and said first terminal of said amplifying cell, and transmits said signal applied to said input terminal to said first terminal of said second amplifying cell in the operation of low gain mode; and wherein said selectively matching circuit changes said input impedance that the power transmitted to said first amplifying cell of the signal applied to said input terminal is to be maximum in the operation of high gain mode, and to be minimum of essentially zero in the operation of low gain mode.
 2. A variable gain low noise amplifier of claim 1, further comprising a short-circuit means connected between said second terminal of said second amplifying cell and said output terminal.
 3. A variable gain low noise amplifier of claim 1, further comprising a short-circuit means connected between said input terminal and said output terminal.
 4. A variable gain low noise amplifier of claim 1, wherein said first amplifying cell comprises first, second, third terminals, amplifying element, resistor and degradation impedance, and wherein said amplifying element, resistor and degradation impedance which are controlled the amounts of current flowed from said first terminal to said second terminal in proportion to the voltage applied to said third terminal; and a first terminal of said amplifying element is formed to said second terminal of said first amplifying cell, said second terminal is connected with one of terminals of said degradation impedance, said third terminal is connected with one of terminals of said resistor and then formed to said first terminal of said first amplifying cell, the other terminal of said resistor is applied to the HG-bias voltage of activating said first amplifying cell in an operation of high gain mode, the other terminal of said degradation impedance is grounded, and said amplifying element is connected to common mode of said second terminal.
 5. A variable gain low noise amplifier of claim 1, wherein said second amplifying cell comprises a first, second, and third terminals; a first amplifying element controlled the amounts of current flowed from said first terminal to said second terminal in proportion to the voltage applied to said third terminal; wherein the second terminal of said first amplifying element is formed to said first terminal of said second amplifying cell, and said third terminal is applied to the LG-bias voltage of activating said second amplifying cell in the operation of low gain mode, and said first amplifying element comprises an amplifying unit connected to common mode of said third terminal; and second and third amplifying element, voltage source, and variable voltage source, which are controlled the amounts of current flowed from said first terminal to said second terminal in proportion to the voltage applied to said third terminal; wherein said first terminal of said second amplifying element is formed to said second terminal of said second amplifying cell, said second terminal is connected to said first terminal of said first amplifying element of said amplifying unit by connecting with said second terminal of said third amplifying element, said third terminal is connected with one of the terminals of said voltage source, said first terminal of said third amplifying element is connected to the power source, said third terminal is connected to said variable voltage source, and the other terminals of said voltage source and variable voltage source are grounded.
 6. A variable gain low noise amplifier of claim 1, wherein said matching circuit comprises a first and second inductor, capacitor, and short-circuit means; and one of said terminals of said first inductor is connected with said second inductor and said capacitor, the other terminal is connected to said short-circuit means, the other terminal of said second inductor is formed first terminal of said matching circuit, the other terminal of said capacitor is formed to second terminal of said matching circuit, and the other terminal of said short-circuit means is grounded. 